Membrane filters with pore sizes much larger than bacteria, 1,2,3,5,8,10, and 14 micra diameter, will effectively remove those bacteria from filtered suspensions. The phenomenon depends on surface charge, intrinsic to membrane filters and probably a by-product of the manufacturing process. Membrane filters of any material possess the charged surfaces, but cellulose and cellulose acetate polymers have more than polycarbonate filters, and thereby are more attractive to bacteria. Gram-positive organisms are attracted more than Gram-negative. Nylon, Teflon, and vinyl membrane filters are far less "magnetic." Bacteria continue to attach to a membrane until the surface charges are neutralized. For a 47 mm diameter cellulosic filter, roughly 1 x 10 to the 9th power bacterial cells are required to "saturate" the filter on continued filtration. Then a much larger percent of the bacteria pass through the filtrate, but never 100% and eventually they pile up on the filter sufficiently to slow filtration by blocking the pores. Only a small percent of the adherent bacteria can be removed by back-washing the filter. No method for quantitative removal that is harmless to bacteria has yet been devised.